Bengaluru: Cyclone Vayu, which put Gujarat on high alert, is the third cyclonic storm of this season of north Indian Ocean cyclones. Last month’s Cyclone Fani and January’s Cyclone Pabuk were the other two storms.
Classified as a ‘very severe cyclonic storm’, Vayu was expected to make landfall in Gujarat, but has now moved in a different direction. Its intensity is also currently weakening. However, strong winds are expected to continue and may cause damage.
The Indian Meteorological Department first observed the cyclone on 9 June, north of Maldives. It then intensified and moved in the north-northwest direction over Arabian Sea.
On 12 June at 5.30 am, the cyclone reached its peak intensity. Sustained winds blew at the speed of 150 kmph. In the Atlantic, this type of a cyclonic storm would be classified as a Category 2 hurricane.
What caused Vayu?
Cyclone Vayu was caused by a phenomenon known as the Madden-Julian Oscillation (MJO). This is a disturbance that propagates eastward and consists of clouds, rainfall, pressure and winds. The entire system then moves fully around the planet in a period of anywhere between one and two months.
There could be multiple MJOs in a season, unlike El Nino. These are termed oscillations as they are wind speeds that move away from the average of the time.
As the disturbance propagates, it does so in two phases. First is the enhanced phase that brings excess rainfall, which is then followed by a suppressed phase that brings a brief dry spell.
Earlier this month, a strong MJO pulse moved east into Indian Ocean, causing storm-like conditions. Starting 9 June, it began to strengthen continuously and steadily, with rapid intensification on the morning of 12 June.
Gujarat escapes Vayu
The storm was moving towards the Gujarat coast, waiting to hit between the regions of Porbandar and Mahuva in the lower curve of the state, but it changed course and is now moving north-westward, parallel to the Gujarat coast. It will still bring extremely heavy rainfall and winds ranging to 145 kmph.
As part of preparations to deal with Cyclone Vayu, Gujarat — which doesn’t see too many storms like this — has already evacuated about 3 lakh people from low-lying areas. Airports in the state have been temporarily shut, while the Army, Navy, Air Force, Coast Guard and Border Security Force have been put on standby.
Arabian Sea storms vs Bay of Bengal storms
The storms on the Arabian Sea side are very different from those on the Bay of Bengal side. In the Arabian Sea, most cyclones do not reach the coast. They are also less deadly than the ones formed in the Bay of Bengal. Besides, the number of cyclones in the Arabian Sea is only about a fifth of those formed in the Bay of Bengal.
In the Bay of Bengal, the ocean’s temperature and salinity balance is constantly disrupted by the flow of freshwater from huge rivers like Brahmaputra and Ganga. The inflow of water from these rivers constantly keeps the top layers of the ocean warm, preventing it from mixing with the colder layers underneath. This gives rise to more frequent conditions conducive to storm formation.
Additionally, the curvature of India’s east coast and the subsequent blocking of winds by the Himalayas and the Western Ghats mountain ranges lead to frequent confinement of storms within the Bay of Bengal.
Also, the higher temperature over the Bay of Bengal causes slow winds in the region. This leads to increased rainfall and humidity. All these cause warm air currents to constantly circulate, thus creating sustained conditions for intense storms.
In the Arabian Sea, however, there are lots of winds blowing over the large, free ocean. The winds constantly move the surface water, helping mix it with other layers underneath and thus dissipating heat and reducing temperature.
Cyclones on this part of the ocean are not too intense, but Vayu coincided with the onset of southwest monsoon this June. The humidity and moisture from the monsoon is providing fodder to Vayu to build up its strength.
But, the longer a cyclone is over the ocean, the stronger it gets. Vayu hasn’t been raging for too long before coming close to the coast, and, therefore, it will not upgrade to a level as severe as Cyclone Fani.